Cochlear implant systems are used to aid patients having a hearing deficiency. More particularly, these systems include a microphone receiving ambient sounds and converting the sounds into corresponding electrical signals, signal processing means for processing the electrical signals and generating cochlear stimulating signals and an electrode assembly for applying the cochlea stimulating signals to the cochlea of an implantee. In response to these electrical stimulations a perception of corresponding ambient sound is elicited in the implantee.
The inner ear of a normally hearing person includes hair cells which convert the displacement of the ear's basilar membrane in response to sound into nervous impulses. Different parts of the basilar membrane of the normal cochlea are displaced maximally by different frequencies of sound so that low frequency sounds maximally displace apical portions whereas higher frequency sounds cause displacement of more basal portions of the membrane. The nervous system is arranged so that a nervous impulse originating from a hair cell located adjacent an apical area of the membrane is perceived as a low frequency sound whereas a nervous impulse originating from a hair cell located adjacent a more basal position of the membrane is perceived as a higher frequency sound. This mapping of position to pitch is well known in the art as the tonotopic arrangement of the cochlea.
In a dysfunctional ear the hair cells may be damaged or absent so that no nervous impulses are generated. In such cases electrical stimulation impulses must be provided artificially to simulate the nervous activity of the hair cells in order to create a perception of sound.
With reference to FIG. 1, a typical cochlear implant is shown, which consists of an external component including a speech processor 1, and an internal component including an implanted receiver and stimulator unit 6 and an intracochlear array 10. The external component further includes a microphone 2 which is shown integral with the speech processor 1. In this illustration the speech processor is arranged so that it can fit behind the ear with the microphone integral therewith. Alternative versions are also envisaged whereby the speech processor is worn on the body and separately attached to the microphone, and also where the speech processor and microphone are implanted in the patient. The present invention is applicable to all these alternatives.
In such cochlear implant devices, ambient sounds are detected by a microphone and a transduced signal is thereby generated, representative of the ambient sound. A processor unit then processes this transduced signal according to one of many strategies, (some of which will be explained further below) and based on this processing stimulation currents are applied between the electrodes of a coupled array. For example, in “monopolar” mode stimulation, stimulation currents may be caused to flow between an electrode of the electrode array 10 and an extracochlear electrode 115. Nervous discharges elicited in the basilar membrane 8 are conveyed to the central nervous system of the wearer by auditory nerve 9.
In the event that the stimulation current flows between an apical electrode such as electrode 111 and extracochlear electrode 115 then a lower pitched hearing sensation will be perceived by a wearer of the prosthesis than will be the case if stimulation current flows between basal electrode 107 and extracochlear electrode 115 because of the previously mentioned tonotopic arrangement of the cochlea. Further pitch information may be transmitted to the wearer corresponding to the rate at which stimulations are delivered.
Many possibilities exist as to the manner in which the signal processing means operates upon the electrical signals in order to produce stimulation signals. However it has been noted in the past that simultaneous stimulation of electrodes is not generally conducive to eliciting a perception of sound that is faithful to the actual acoustic signals being processed. This is because if electrodes are stimulated simultaneously, current paths between electrodes can interact, causing undesirable stimulation. Consequently most cochlear prosthesis stimulation strategies stimulate by means of only one electrode at a time.
In the past designers of cochlear implant stimulation strategies have striven to optimise the intelligibility of spoken words as perceived by the wearer of a cochlear implant.
One of the earliest sound processing and cochlear stimulation strategies is described in U.S. Pat. No. 4,532,930 to the present applicant. In that patent there is taught the use of a filter (F0) dedicated to extracting the voice pitch of a speech signal. The periodicity of the voice pitch is used to set the stimulation periodicity for two or three electrodes. A second, and possibly third, channel is analysed to determine periodicity and amplitude in a selected frequency band.
The periodicity extracted from the second filter, and possibly third filter, is used to select which electrode is stimulated for the second and third channels while the periodicity of stimulation of the channel is determined by the periodicity of the output signal from the F0 filter.
Another stimulation arrangement is described in U.S. Pat. No. 4,207,441. In that system there are n electrodes each coupled to one of n filters. Each electrode is stimulated once per analysis period, with an intensity corresponding to the amplitude of the corresponding filter channel. The analysis period of this system is predetermined and hence is not related to the signal on the filter outputs.
More recently in EP 0 745 363 there is described a stimulation system which takes into account the temporal behaviour of the cochlea. In an embodiment of the invention therein described a wavelet transformation is used to extract the temporal information with a view to using this information to determine the sequence of stimulation of the electrodes. The purpose of the invention is to improve the temporal resolution in response to a rapidly changing audio spectrum.
A problem that has been faced by users of cochlear implants featuring prior art stimulation schemes is that while intelligibility of spoken words is often good the user's pitch perception, and in particular perception of music, is poor. Accordingly, it is an object of the present invention to provide an apparatus and method for use in a multi-channel cochlear implant which will improve a user's perception of pitch.